The majority of modern cable telecommunications systems used today are built with a Hybrid Fiber Coax (HFC) network topology. This topology uses fiber optic cable to transmit optical signals to and from a fiber optic node located near a cable subscriber, such as a residential home subscribing to cable telecommunication services. The fiber optic node receives and converts the optical signals into Radio Frequency (RF) signals. These RF signals are then transmitted from the fiber optic node to the subscriber's home over a coaxial cable.
FIG. 1 illustrates a conventional HFC network 100. The HFC network 100 includes a head-end 102. The head-end 102 is a facility for receiving, processing, and distributing media signals, including video, audio, and data signals, over the HFC network 100. The head-end 102 is typically maintained or managed by a media service provider or a multiple service operator (MSO), such as a cable television (CATV) provider or an Internet service provider (ISP). The head-end 102 may include any reasonably suitable electrical equipment for receiving, storing, and re-transmitting media signals, such as media servers, satellite receivers, modulators/demodulators, edge decoders, etc.
The head-end 102 typically includes a transmitter to transmit the media signals downstream to the subscribers 110, over a fiber optic link 104 to one or more fiber optic nodes 106, each supporting any number of subscribers 110, depicted here as residential homes. Each fiber optic node 106 receives and converts the optical signals sent from the head-end 102 into RF signals, which are then delivered to the subscribers 110, via coaxial cables 108. The subscribers 110 have a receiver, such as a cable modem and/or a multimedia terminal adapter (MTA), for receiving the signals sent from the transmitter.
Since information transmitted from the head-end 102 is often expressed in a digital form, for example as a stream of finite values of data, head-end transmitters often send digitally modulated signals. Various modulation techniques have been developed to efficiently transmit information expressed in a digital form. These include amplitude modulation and phase modulation. For example, quadrature phase shift keying (QPSK), DQPSK, and M-level quadrature amplitude modulation (QAM) are a few such techniques. These techniques define a constellation of symbols, where each symbol may be used to communicate a plurality of bits of data. The symbols are identified based on their position on a plot of phase and amplitude called an I/Q plane.
When modulated signals are transmitted downstream from the transmitter at the head-end 102 to a receiver, generally, the initial signal created at the head-end 102 is pure. That is, the modulated signal originating from the head-end 102 lacks degradations and/or impairments. Impairments are errors gathered by a signal as the signal is transmitted. Impairments inhibit the ability of a receiver to interpret the signal sent from the transmitter and may sometimes lead to failure of the receiver and loss of data.
Impairments can be attributed to a variety of sources, and they can distort signal transmission on the HFC network 100. For instance, many devices, including common household appliances such as garbage disposals and blenders, emit signals at various frequencies, which enter the HFC network 100 through poorly shielded cables or through the communication devices attached to the cable network within the home.
Moreover, additional traffic on networks, such as signals from other receivers and television signals, can interfere with signal transmission to create impairments. The accumulation of interfering signals reduces the carrier-to-noise ratio (CNR), which, in turn, reduces throughput as forward error correction (FEC) techniques are more heavily utilized to deal with signal errors caused by impairments.
One particular impairment is known as hum modulation, also known as amplitude modulation (AM) hum, which is the amplitude distortion of a signal caused by the modulation of the signal by components of the frequency on the transmission line that carries the signal. In other words, AM hum in a system signal is the peak-to-peak variation in the signal level caused by undesired low frequency disturbances (hum or repetitive transients) generated within the system, or by inadequate low frequency response. AM hum may be expressed as a ratio, in decibels (dB), of the peak-to-peak variation of the transmitting signal to the peak of such a signal. Alternatively, AM hum may be expressed as a percentage of the peak-to-peak variation of the signal level to peak voltage amplitude of the signal.
Impairments, such as AM hum have plagued HFC networks since their inception, but regardless of how well a network is designed, impairments remain and can negatively affect system performance. A key to efficient transmission of data over a network is accounting for these impairments in the design of the receiver. For example, cable modems are designed with filters, which resolve some impairments in signals received from the head-end 102. However, while filters currently improve signal transmission, impairments, such as AM hum, continue to inhibit the transmission of data over communications networks, such as the HFC network 100.